Nuts & Volts

■ FIGURE 4. Testing
LED Polarity.
■ FIGURE 5. Testing
Your Work.
■ FIGURE 6. Header
Attachment.
■ FIGURE 7. Hot
Glue Strain Relief.
plastic (see Figure 3) at each end that
kept them from coming apart and,
once that was cut off with a pair of
scissors, it was easy to separate them
into mono cables, and then cut two
feet from each end.
I discovered they weren't actually
shielded and the ground wire was
a little delicate to work with, but I
ended up with four two-foot power
cords for less than $1.
The rest of the parts are from
Jameco (www.jameco.com). If you
substitute LEDs, make sure you
get small ones (T 3/4) and verify the
voltage drop for various currents to
redo the design computations above,
if necessary.
Since it has a much better price
per pin, I bought the 17 pin headers
(SMH17 = Jameco #103376CX) and
cut off two eight-pin sections.
Double-sided, plated through,
solder coated circuit boards are available
from my store (http://Impossible
Enterprises.com/) for $6.50 each or
with a kit of parts for $9.50. Prices and
availability may vary.
PC Board
I used software from
ExpressPCB (www.expresspcb.com)
to lay out the schematics and board
and their service to manufacture
them. Please note that the bottom
copper is still viewed from the top,
and will need to be reversed left-toright
if you are making your own
boards. A dremel-like tool with a
thin cutting disk was used to saw the
large PCB into individual modules.
All of the design files are available
on the Nuts & Volts website (www.
nutsvolts.com), as well as in my
store.
The gain on these devices is
incredible. When near the threshold
voltage, a very small voltage change
will make a very large change in the
output, but inverted. Unintentional
feedback can lead to oscillation,
so there must be shielding between
the input and output sections of
these devices. For the
most part, this isn't
much of a problem
here, as it's just us
humans who are
watching the LEDs
and we'll never notice
if they flicker when a
pin is driven really
near 2V (the reference
voltage). But, it may
be the case that a
really strong oscillation
will generate a
signal in the power
leads or even back
down into the circuit
under test so, in
the layout, I tried to
get a band of ground
between the inputs
■ FIGURE 8. The Entire
Octal Logic Probe.
40 April 2006